Moulding unit for a plant for blow-moulding plastic containers, particularly bottles

ABSTRACT

A plant for blow-moulding plastic containers ( 2 ), particularly bottles, from respective parisons ( 3 ) comprises at least one moulding unit ( 15 ) composed of two half-moulds ( 17   a   , 17   b ), each mobile relative to the other half-mould ( 17   a   , 17   b ) between a position of opening and a position of closing at least two cavities ( 19 ) for blow-moulding containers ( 2 ); the two half-moulds ( 17   a   , 17   b ) being oriented in such a way that they close in a closing plane ( 20 ) that is substantially tangent to the feed path (P) of the moulding unit ( 15 ).

TECHNICAL FIELD

This invention relates to a moulding unit for a plant for blow-moulding plastic containers, particularly bottles.

BACKGROUND ART

In the bottling industry, prior art plant for making plastic bottles comprises: a blow-moulding wheel mounted in such a way as to rotate about its longitudinal axis and equipped with at least one blow-moulding unit, which is fed by the blow-moulding wheel along a circular path extending around this axis and which comprises two half-moulds, each mobile relative to the other half-mould between a position of opening and a position of closing at least two cavities for blow-moulding bottles from respective parisons.

The two half-moulds are oriented in such a way that they are closed in a closing plane that is substantially radial with respect to the feed path of the moulding unit.

Prior art moulding units of this type have several drawbacks due mainly to the fact that inserting two parisons into a moulding unit with radial closing plane and then extracting two bottles from this type of moulding unit are relatively complex and difficult operations.

Also known in the prior art, from document DE102004057102, is a moulding unit where the half-moulds define a closing plane that is substantially tangent to the path followed by the moulds. In the solution described in that document, the half-mould on the outside is movable while the half-mould on the inside remains stationary (relative to the carousel it is associated with), that is to say, it does not move relative to the half-mould on the outside.

This solution, too, is not free of disadvantages, however, since the effectiveness and convenience of the closure at the bottom of the mould (that is, of the container moulding unit) are not entirely satisfactory. When the half-moulds are in the closed position, they form a hole at the bottom which is operatively closed by a base-mould.

As a result, the fact that one of the two half-moulds never moves relative to the other half-mould makes it difficult to move the base-mould between a mould closed position and a mould open position where the hole at the bottom is left free.

Moreover, the need to move the base-mould means that the form of the half-moulds and of the base-mould cannot be optimized in such a way as to close the mould hermetically, precisely because the base-mould must be moved away from the half-moulds without moving the inside half-mould. From patent document US2008/143022 it is known a method for controlling the opening and closing of a book-type blow mould for producing thermoplastic containers. The mould has two mould halves which can pivot through different angles. However, the closing plane of the moulds is radial with respect to the feed path of the moulding unit; therefore, this technical solution has the aforementioned drawbacks.

From patent document US2008/063742 it is known a blow moulding machine for the manufacture of containers based on performs. The moulds of said machine comprise a mould base which can slide vertically to open and close the bottom of the mould. However, also according to US2008/063742 the closing plane of the moulds is radial with respect to the feed path of the moulding unit.

Document WO2008/110887 of the same Applicant describes compression moulds for compression moulding of plastics in a pasty state, provided with means for heating said mould. Said mould comprises two half moulds, each having a corresponding bottom element with a tapered surface. However, this document does not regard blow moulding.

AIM OF THE INVENTION

This invention has for an aim to provide a moulding unit for a plant for blow-moulding plastic containers, particularly bottles, that is free of the above mentioned disadvantages.

In particular, this invention has for an aim to provide a moulding unit for a plant for blow-moulding plastic containers that allows blow-moulding of two or more bottles in a particularly fast and efficient manner while facilitating demoulding of the bottles and movement of the base-mould.

Another aim of the invention is to facilitate extraction of the blow-moulded bottles from the mould.

Accordingly, this invention provides a moulding unit for a plant for blow-moulding plastic containers, particularly bottles, as described in the appended claims.

More specifically, the invention provides a moulding unit (that is, a mould) for a blow-moulding plant (that is, a blow-moulding machine) for making plastic containers, particularly bottles, from respective parisons, the moulding unit being movable along an annular path (P) and comprising two half-moulds positioned one on the inside and the other on the outside of the path (P), the half-mould on the outside being movable about a substantially vertical pivot axis relative to the half-mould on the inside between a position of opening and a position of closing at least two cavities for blow-moulding respective containers; the two half-moulds defining a closing plane that is substantially tangent to the path (P).

According to the invention, both the half-moulds are movable through different angles between respective open and closed positions.

The fact that both the half-moulds are movable makes it easier to extract the base-mould and the fact that each half-mould is movable through a different angle compared to the other half-mould speeds up mould opening and closing operations and makes the mould and the blow-moulding machine particularly efficient.

In particular, the angle through which the half-mould on the inside moves is smaller than the angle through which the half-mould on the outside moves.

This facilitates bottle demoulding, thereby optimizing the speed of opening and closing operations and maximizing the efficiency of the mould and of the blow-moulding machine.

The moulding unit comprises a closing element that is vertically movable between a raised position where it is partly inserted between the two half-moulds in the closed position to form a mould bottom wall, and a lowered position where it does not impinge on the half-moulds so that the container can be extracted from the mould after blow-moulding.

The closing element comprises an upper portion whose horizontal cross section is greater than an opening formed at the bottom of the two half-moulds when they are in the closed position.

More specifically, the upper portion of the closing element forms an annular surface shaped to match portions of the inside surface of the half-moulds, said matching surfaces being designed to hermetically seal the opening at the bottom of the half-moulds when they are in the closed position.

The matching surfaces have at least one downwardly converging tapered portion.

The invention also provides a blow-moulding machine comprising a carousel and a plurality of the moulding units associated with the carousel.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the accompanying drawings which illustrate a preferred, non-limiting embodiment of it and in which:

FIG. 1 is a schematic plan view, with some parts cut away for clarity, of a preferred embodiment of the plant according to the invention;

FIG. 2 is a schematic plan view, with some parts enlarged and others cut away for clarity, of a first detail of the plant of FIG. 1;

FIG. 3 is a schematic plan view, with some parts cut away for clarity, of a first detail from FIG. 2, shown in two different operating conditions;

FIG. 4 is a schematic plan view, with some parts cut away for clarity, of a detail from FIG. 3, shown in four different operating conditions;

FIG. 5 is a schematic side view, with some parts cut away for clarity, of a second detail from FIG. 2, shown in two different operating conditions;

FIG. 6 is a schematic side view, with some parts cut away for clarity, of a third detail from FIG. 2;

FIG. 7 is a schematic plan view, with some parts cut away for clarity, of a fourth detail from FIG. 2;

FIG. 8 is a schematic plan view, with some parts cut away for clarity, of a second detail of the plant of FIG. 1;

FIG. 9 is a schematic side view, with some parts cut away for clarity, of the detail of FIG. 8; and

FIG. 10 is a schematic plan view, with some parts cut away for clarity, of another embodiment of the detail of FIGS. 8 and 9;

FIG. 11 is a perspective view of the detail illustrated in FIG. 3;

FIG. 12 is a top view of the detail illustrated in FIG. 3;

FIG. 13 is a cross section of the detail illustrated in FIG. 3;

FIG. 14 illustrates the detail A from FIG. 13.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

With reference to FIG. 1, the numeral 1 denotes in its entirety a plant for blow-moulding plastic containers, in this particular case, plastic bottles 2 (FIG. 9), from respective parisons 3 of known type (FIG. 5), each of which comprises an elongated cup-shaped body 4 having an externally threaded open end 5, and an annular neck 6 extending radially outwards from the outside surface of the body 4 itself.

The plant 1 comprises a blow-moulding machine 7 for blow-moulding the bottles 2, a line 8 for feeding the parisons 3 to the machine 7, and a line 9 for feeding the bottles 2 from the machine 7 to a customary filling machine 10.

As illustrated in FIGS. 1 and 2, the machine 7 comprises a blow-moulding wheel (or carousel) 11 mounted in such a way as to rotate continuously (counterclockwise in FIGS. 1 and 2) about its longitudinal axis 12, which is substantially vertical and at right angles to the drawing plane of FIGS. 1 and 2. The wheel is connected to the lines 8 and 9 at a first and a second transfer station 13, 14, respectively, and is equipped with a plurality of moulding units 15 which are mounted round the edge of the wheel 11, are uniformly distributed around the axis 12 according to a predetermined spacing, and are advanced by the wheel 11 along a circular path P around the axis 12 and through the stations 13, 14.

Each unit 15 comprises an interchangeable mould 16 comprising, in the embodiment illustrated in FIGS. 2 and 3, two half-moulds 17, each hinged to the wheel 11 to rotate relative to the wheel 11, under the action of a drive device (not illustrated) about a substantially vertical pivot axis 18 parallel to the axis 12 between a position for opening (FIG. 3 a) and a position for closing (FIG. 3 b) two moulding cavities 19, each having the shape of a bottle 2 and a longitudinal axis 19 a parallel to the axis 18, being open to the outside at a hole at the top of it smaller in diameter than the neck 6 of a parison 3, and acting in conjunction with a pneumatic device of known type, not illustrated, designed to blow compressed air into the parison 3 inside the cavity 19 to mould the respective bottle 2.

The two half-moulds 17 are oriented in such a way that they are closed in a closing plane 20 substantially tangent to the path P and are locked in position by a locking device 21 comprising a cylindrical rod 22 having a longitudinal axis 23 parallel to the axis 12, extending through one of the two half-moulds 17 (hereinafter denoted by the reference 17 a) along a vertical line 24 and being rotatably coupled to the half-mould 17 a to rotate, relative to the half-mould 17 a, about the axis 23 itself.

The rod 22 comprises at least one undercut portion 22 a limited by a flat face parallel to the axis 23, and has keyed to it a rocker arm 25 mounted coaxially with the axis 23 and in turn comprising two arms 26, 27 which extend radially outwards from the axis 23 itself.

The device 21 also comprises a latch 28, elongated in shape, which extends transversally to the line 24, is fixed to the other half-mould 17 (hereinafter denoted by the reference 17 b), and has an indent 29 formed on the latch 28 along the line 24 to receive and retain the portion 22 a.

During the movement of the unit 15 from the station 13 to the station 14, the two half-moulds 17 a, 17 b are in their closed position, and a spring 30, interposed between the arm 26 and the half-mould 17 a holds the rod 22 and the latch 28 in a normally locked position (FIG. 4 a), where the portion 22 a engages the indent 29 to prevent the mould 16 from opening.

During the movement of the unit 15 from the station 14 to the station 13, the rod 22 is moved against the action of the spring 30 to a released position (FIG. 4 b), where the portion 22 a disengages the indent 29 to enable the two half-moulds 17 a, 17 b to move (FIGS. 4 c and 4 d) to the open position by engaging a tappet roller 31 mounted on the arm 27 in a cam (not illustrated) designed to control the angular position of the rocker arm 25 around the axis 23.

In connection with the above, it should be noted that the two half-moulds 17 a, 17 b move relative to each other through different angles between the open and closed positions of the mould 16. More specifically, the angle of movement of the half-mould 17 located radially towards the outside of the path P, namely, the half-mould 17 b, is greater than the angle of movement of the half-mould 17 located radially towards the inside of the path P, namely, the half-mould 17 a.

With reference to FIGS. 11-14, attention is drawn to the following.

The moulding unit 15 (that is, the mould 15) comprises a closing element 501 (also referred to as base-mould) designed to close the bottom of the mould 15.

When the half-moulds 17 are in the closed position, they form a closed side wall of the mould 15 but leave an opening at the bottom of the mould 15.

The closing element 501 is vertically movable between a raised position and a lowered position.

In the raised position, the closing element 501 is partly inserted between the half-moulds 17 in the closed position to form a bottom wall of the mould 15.

In the lowered position the closing element 501 does not impinge on the half-moulds so that the container can be extracted from the mould after blow-moulding.

It should be noted that the closing element 501 is also designed to support the blow-moulded containers and to form a bottom portion 502 of the moulding surface.

The closing element 501 comprises an upper portion 503 shaped in such a way as to impinge on an edge of an opening formed at the bottom of the half-moulds 17 when they are in the closed position. In other words, if one tried to move the closing element 501 from the lowered to the raised position or vice versa without opening the half-moulds 17, an upper portion of it 503 would impinge on the edge of the hole, thereby preventing the movement of the closing element 501 from one position to the other.

Preferably, the upper portion 503 of the closing element 501 forms an annular surface 504 shaped to match portions 505 of the inside surface of the half-moulds 17, said matching surfaces being designed to hermetically seal the opening at the bottom of the half-moulds 17 when they are in the closed position.

The matching surfaces 504 and 505 preferable have at least one downwardly converging tapered portion.

This optimizes the seal of the mould 15.

According to the invention, the movement of the closing element 501 from the lowered to the raised position and vice versa is particularly easy because both the half-moulds 17 are movable away from each other to make the hole larger.

With reference to FIG. 5, the unit 15 also comprises a stretching unit 32, mounted above the mould 16 and, in this particular embodiment, comprising three stretching rods 33 parallel to each other and to the line 24, two of which (hereinafter denoted by the reference 33 a) are mounted in a position coaxial with the cavities 19, and the other (hereinafter denoted by the reference 33 b) extends between the rods 33 a.

The rods 33 a, 33 b are slidably coupled to both a mounting bracket 34 fixed to the wheel 11, and to a mounting plate 35 that is in turn coupled by a lead nut and screw mechanism to an output shaft 36 of an electric motor 37 fixed to the bracket 34, and are selectively locked on the bracket 34 or on the plate 35 along the line 24 by means of customary fastening screws not illustrated.

As illustrated in FIG. 5 a, when the moulds 16, that is to say, the moulds with two moulding cavities 19, are mounted on the wheel 11, the rod 33 b is locked axially on the bracket 34 and the rods 33 a are locked axially on the plate 35 and are driven by the motor 37 along the line 24 between respective raised positions (FIG. 5 a), where the rods 33 a are positioned substantially outside the respective parisons 3, and respective lowered positions (not illustrated), where the rods 33 a engage the respective parisons 3 in such a way as to stretch them axially along the line 24.

As illustrated in FIG. 5 b, when the moulds 16 are replaced with moulds (not illustrated) having a single central moulding cavity each, for blow-moulding a bottle 2 of larger size, the rods 33 a are locked axially on the bracket 34 and the rod 33 b is locked axially on the plate 35 and is driven by the motor 37 along the line 24 between a raised position (FIG. 5 b), where the rod 33 b is positioned substantially outside the respective parison 3, and a lowered position (not illustrated), where the rod 33 b engages the respective parison 3 in such a way as to stretch it axially along the line 24.

It may be inferred from the foregoing that changing the moulds of the blow-moulding units 15 does not necessitate changing the related stretching units 32 and that setup times are therefore relatively short.

In an embodiment that is not illustrated, when the wheel 11 mounts the moulds 16, that is to say, the moulds with two moulding cavities 19, the rod 33 b must be removed and, when the moulds 16 are replaced with moulds (not illustrated) having a single central moulding cavity each, the rods 33 a must be removed.

With reference to FIGS. 1 and 2, the feed line 8 comprises a heating device 38 for thermally conditioning the plastic material the parisons 3 are made of to a temperature greater than their glass transition temperature, and a transfer wheel 39 connected to the device 38 at a transfer station 40 and to the blow-moulding wheel 11 at the station 13.

The device 38 comprises an endless chain conveyor 41 that is trained around two pulleys 42 (only one of which is illustrated in FIGS. 1 and 2) rotatably mounted to turn about respective longitudinal axes 42 a parallel to the line 24, extends through at least one customary oven not illustrated, and mounts a plurality of pick-up and transporting members 43 uniformly distributed along the conveyor 41 and advanced by the conveyor 41 itself along an annular path S.

As illustrated in FIGS. 2 and 6, each member 43 comprises a tubular guide element 44 that is hooked up to the conveyor 41, extends along the line 24, is held in the correct position by engagement with a tappet roller 45 at a cam 46, and is slidably engaged by a respective pick-up rod 47 whose longitudinal axis 47 a is parallel to the line 24.

The rod 47 is equipped with a plurality of balls 48 that are uniformly distributed around the axis 47 a, are seated in respective housings 49 formed radially on a bottom end of the rod 47 protruding to the outside of the element 44, and protrude radially outwards from the respective housings 49 under the pushing action of respective springs 50 mounted inside the respective housings 49 transversally to the line 24.

The rod 47 is mobile under the pushing action of a tappet roller 51 placed in contact with a cam 52, along the line 24 between a lowered position (not illustrated), where the rod 47 extends inside the respective parison 3 to allow the balls 48 to grip the parison 3 under the pushing action of the respective springs 50, and a raised position (FIG. 6), where the rod 47 disengages the parison 3.

The member 43 further comprises an interchangeable limit stop block 44 a that protrudes downwardly from the element 44 coaxially with the axis 47 a to come into contact with the end 5 of the parison 3, is removably mounted on the element 44, and can be replaced according to the size and/or shape of the end 5 itself.

With reference to FIGS. 2 and 7, the wheel 39 comprises a drum 53 mounted to turn continuously about its substantially vertical longitudinal axis 54 parallel to the line 24, and a plurality of pick-up and transporting units 55 (in this particular embodiment six pick-up and transporting units 55) that are mounted along a peripheral edge of the drum 53, protrude radially outwards from the drum 53, and are advanced by the drum 53 itself around the axis 54 and through the stations 13 and 40.

Each unit 55 comprises a supporting rocker arm 56 that is hinged to the drum 53 to turn relative to the drum 53 itself about a pivot axis 57 substantially parallel to the line 24, and is equipped, at its first arm 58, with a tappet roller 59 in contact with a cam 60 designed to control the angular position of the rocker arm 56 around the axis 57 itself.

The rocker arm 56 also comprises a second, elongated arm 61 slidably engaged by a slider 62 that is equipped with a tappet roller 63 in contact with a cam 64 designed to control the position of the slider 62 along the arm 61, and supports at a free end of it, protruding to the outside of the arm 61 itself, a pick-up unit 65 comprising a rocker arm 66 hinged to the slider 62 to turn relative to the slider 62 and under the action, in this embodiment, of an electric motor 67 mounted on the slider 62, about a pivot axis 68 parallel to the line 24.

The rocker arm 66 has two arms 69, 70, opposite each other. The arm 69 mounts a pick-up element 71 having the shape of a fork and comprising two arms 72 which are mounted to oscillate about respective pivot axes 73 parallel to the line 24 between a position of releasing (not illustrated) and a position of clamping (FIG. 7) a respective parison 3, and which a spring 74 interposed between the arms 72 normally holds in the clamping position where the arms 72 grip the parison 3 above the neck 6.

The arm 70 mounts two pick-up elements 75 entirely equivalent to the element 71, parallel to each other and positioned side by side, having concavities facing away from the concavity of the element 71, and further having respective longitudinal axes 75 a that are parallel to the line 24 and spaced from each other at a distance D1 equal to the distance D2 between the longitudinal axes 19 a of the two moulding cavities 19 of one mould 16.

In connection with the above, it should be noted that:

according to the number of moulding cavities 19 in each mould 16, the pick-up elements 71, 75 are selectively oriented by the respective electric motors 67 about the respective axes 68 between respective operating positions where the pick-up elements 71, 75 are oriented towards the outside of the wheel 39, and respective rest positions where the pick-up elements 71, 75 are oriented towards the inside of the wheel 39;

by combining the movements of the rocker arms 56 about the respective axes 57 with the movements of the sliders 62 along the respective arms 61 and with the movements of the rocker arms 66 about the respective axes 68, the elements 71, 75 in their operating positions are kept substantially parallel to the path P at the station 13 and parallel to the path S at the station 40; and

the station 40 for transferring the parisons 3 from the conveyor 41 to the wheel 39 is positioned along a curved stretch T of the path S, preferably a stretch T around one of the pulleys 42, where the spacing P1 between the rods 47, and hence between the parisons 3, is equal to the distance D1 and hence to the distance D2, and different from the spacing between the rods 47 and, hence between the parisons 3, along the straight stretches of the path S.

From the above it may be inferred that the position of the station 40 and the shape of the wheel 39 make it possible to set up the plant 1 quickly and easily not only using moulds 16 with two moulding cavities 19 but also using moulds (not illustrated) with a single moulding cavity.

In another embodiment that is not illustrated, the rocker arms 66 are eliminated and replaced with interchangeable pick-up units, each of which is hinged to the respective slider 62 to oscillate about the respective axis 68 under the control of a tappet roller engaged in a cam, and which is equipped with one pick-up element 71 or two pick-up elements 75.

As illustrated in FIG. 1, the feed line 9 comprises: a transfer wheel 76 that is connected with the wheel 11 at the station 14, is entirely equivalent to the wheel 39 and will not therefore be described in further detail; a wheel 77 for extracting the bottles 2 from the pick-up elements 71, 75 of the transfer wheel 76; and a train 78 of feed wheels 79 connected to the wheel 77 through an interposed dispensing device 80 and designed to feed the bottles 2 to the filling machine 10.

With reference to FIG. 8, the wheel 77 is mounted to rotate continuously about its longitudinal axis 81 parallel to the line 24, and is equipped with a plurality of semi-cylindrical pockets 82 formed along a peripheral edge of the wheel 77, open radially towards the outside, each designed to receive and retain a bottle 2, and divided into a plurality of groups 83 of pockets 82 equally spaced around the axis 81 and each comprising, in this particular embodiment, three pockets 82, of which one (hereinafter denoted by the reference 82 a) is positioned between the other two (hereinafter denoted by the reference 82 b) in turn positioned from each other at a distance D3 equal to the distances D1 and D2.

Each wheel 79 is mounted to rotate continuously about its longitudinal axis 84 parallel to the line 24, and is equipped with a plurality of semi-cylindrical pockets 85 formed along a peripheral edge of the wheel 79, open radially towards the outside, each designed to receive and retain a bottle 2, and uniformly distributed around the axis 84 according to a spacing P2 equal to the spacing of the dispensing nozzles (not illustrated) of the filling machine 10.

In this connection it should be noted that the spacing P2 is greater than the distance D3 and less than a distance D4 between two adjacent pockets 82 a and that the distance D4 is also equal to the spacing between the moulds 16 on the blow-moulding wheel 11 and is therefore different from the distance D2.

As illustrated in FIGS. 8 and 9, the dispensing device 80 comprises, in this embodiment, a screw feeder 86, that is mounted to rotate continuously about its longitudinal, substantially horizontal axis 87 transversal to the line 24, extends between the wheel 77 and the infeed wheel 79 (hereinafter denoted by the reference 79 a) of the train 78 of wheels 79, has two starts 88, that is to say, a number of starts equal to the number of moulding cavities 19 of one mould 16, is designed to engage the bottles 2 above the respective necks 6, and acts in conjunction with a guide channel 89 which extends parallel with the axis 87, which is slidably engaged by the bottles 2 and which is delimited by two side walls 90 designed to support the bottles 2 under the respective necks 6.

The embodiment illustrated in FIG. 10 differs from the one illustrated in the drawings described up to now only in that:

the moulds 16 with two moulding cavities 19 are eliminated and replaced with moulds (not illustrated) having only one moulding cavity;

the pick-up elements 71, 75 operate and come to rest at positions different from the above; and

the screw feeder 86 is eliminated and replaced with a screw feeder 91 having a single start 92.

In an embodiment that is not illustrated, the dispensing device 80 is eliminated and replaced with a customary, variable spacing transfer wheel.

The operation of the plant 1 will now be described with reference to the moulds 16 with two moulding cavities 19 and starting from an instant where the pick-up elements 71, 75 of the transfer wheels 39, 76 have been moved to their rest and operating positions, respectively.

The conveyor 41 feeds the parisons 3 one after the other to the station 40 where the spacing P1 of the parisons 3 is equal to the distance D1 between the pick-up elements 75 of each pick-up unit 65 of the wheel 39 so that each unit 65 can pick up a pair of parisons 3 from the conveyor 41.

Each pick-up unit 65 is then advanced by the wheel 39 through the transfer station 13 in step with a respective mould 16, whose half-moulds 17 a, 17 b are opened at the station 14 after the respective rod 22 has been moved to the released position in order to drop the parisons 3 into the respective moulding cavities 19.

At this point, the two half-moulds 17 a, 17 b are closed; the rod 22 is moved by the spring 30 to the latch 28 locked position; and the bottles 2 are moulded as the mould 16 advances from the station 13 to the station 14, where the mould 16 is opened to enable the pick-up elements 75 of one pick-up unit 65 of the transfer wheel 76 to extract the bottles 2 just moulded.

After that, the bottles 2 are first transferred by the wheel 76 into the pockets 82 b of one group 83 of pockets 82 on the extraction wheel 77, are then advanced by the wheel 77 to the infeed end of the channel 89 and by the screw feeder 86 along the channel 89 and, lastly, are transferred by the screw feeder 86 into the pockets 85 of the wheel 79 a according to the aforementioned spacing P2.

Operation of the plant 1 with moulds (not illustrated) having a single moulding cavity involves simply replacing the moulds 16 with the other moulds, moving the pick-up elements 71, 75 of the transfer wheels 39, 76 to their operating and rest positions, respectively, and replacing the screw feeder 86 with the screw feeder 91. It should be noted that, in this case, each bottle 2 is first transferred by the wheel 76 into the pocket 82 a of one group 83 of pockets 82 on the extraction wheel 77, is then advanced by the wheel 77 to the infeed end of the channel 89 and by the screw feeder 91 along the channel 89 and, lastly is transferred by the screw feeder 91 into a pocket 85 of the wheel 79 a according to the aforementioned spacing P2.

From the above it may be inferred that the position of the station 40, the shape of the transfer wheels 39, 76 and the presence of the screw feeders 86, 91 make it possible to set up the plant 1 quickly and easily not only using moulds 16 with two moulding cavities 19 but also using moulds (not illustrated) with a single moulding cavity, and to feed the same filling machine 10 and according to the same spacing P2 with bottles 2 of different sizes made in moulds 16 with two moulding cavities 19 as well as in moulds (not illustrated) with a single moulding cavity.

Lastly, it should be noted that, preferably, the assembly composed of the blow-moulding wheel 11, the transfer wheels 39, 76, the extraction wheel 77 and the screw feeders 86, 91 is driven by one drive motor (not illustrated), while the chain conveyor 41 and the train 78 of feed wheels 79 may be driven either by a single drive motor (not illustrated) or, alternatively, by separate motors (not illustrated) synchronized with each other. Indeed, since the use of moulds 16 with two moulding cavities 19 confers on the blow-moulding machine 7 a production capacity that is twice the production capacity conferred on the blow-moulding machine 7 by the use of moulds (not illustrated) with a single moulding cavity, the feed speeds of the conveyor 41 and of the train 78 of feed wheels 79 are selectively controlled in such a way that when the machine 7 mounts moulds 16 with two moulding cavities 19 the speeds are substantially twice the speeds imparted to the conveyor 41 and to the train 78 of feed wheels 79 when the machine 7 mounts moulds (not illustrated) with a single moulding cavity. 

The invention claimed is:
 1. A moulding unit for a plant for blow-moulding plastic containers (2), particularly bottles, from respective parisons (3), the moulding unit being movable along an annular path (P) and comprising two half-moulds (17 a, 17 b) positioned one on the inside and the other on the outside of the path (P), the half-mould (17 b) on the outside being movable about a substantially vertical pivot axis (18) relative to the half-mould (17 a) on the inside between a position of opening and a position of closing at least two cavities (19) for blow-moulding respective containers (2); the two half-moulds (17 a,17 b) defining a closing plane (20) that is substantially tangent to the path (P), wherein both half-moulds (17 a, 17 b) are movable away from each other through different angles between respective open and closed positions, the angle of movement of the half-mould (17 b) on the outside towards the outside of the path (P) being greater than the angle of movement of the half-mould (17 a) on the inside towards the inside of the path (P), wherein the half-mould (17 a) on the inside of the path (P), in the respective open position, is rotated a predetermined angle apart from a position of finished blow moulded articles, inwardly with respect to said closing plane (20) that is substantially tangent to the path (P).
 2. The moulding unit according to claim 1, comprising a closing element (501) that is vertically movable between a raised position where it is partly inserted between the half-moulds (17 a, 17 b) in the closed position to form a mould bottom wall, and a lowered position where it does not impinge on the half-moulds (17 a, 17 b) so that the container (2) can be extracted from the mould after blow-moulding.
 3. The moulding unit according to claim 2, wherein the closing element (17 a, 17 b) comprises an upper portion (503) shaped in such a way as to impinge on an edge of an opening formed at the bottom of the half-moulds (17 a, 17 b) when they are in the closed position.
 4. The moulding unit according to claim 3, wherein the upper portion (503) of the closing element (501) forms an annular surface (504) shaped to match portions (505) of the inside surface of the half-moulds (17 a, 17 b), said matching surfaces being designed to hermetically seal the opening at the bottom of the half-moulds (17 a, 17 b) when they are in the closed position.
 5. The moulding unit according to claim 4, wherein said surfaces (504, 505) have at least one downwardly converging, tapered portion.
 6. The moulding unit according to claim 1, comprising a mould (16) defining two moulding cavities (19).
 7. A blow-moulding machine comprising a carousel and a plurality of moulding units associated with the carousel, wherein said moulding units are moulding units according to claim
 1. 8. The moulding unit according to claim 1, wherein the two half-moulds (17 a, 17 b) have corresponding contact surfaces, said contact surfaces being in contact with each other to define the closing plane in the closing position, wherein the half-mould (17 a) on the inside of the path (P), in the respective open position, has its contact surface rotated inwardly, with respect to the closing plane (20) tangent to the path (P), of the angle of movement of said half-mould (17 a) on the inside of the path (P), relative to the direction of rotation of the mould along the path (P).
 9. The moulding unit according to claim 1, wherein the moulding unit comprises a vertical shaft defining said pivot axis (18), and wherein the two half-moulds (17 a, 17 b) have corresponding ends configured to partially embrace said shaft, whereby each one of the two half-moulds (17 a, 17 b) is hinged to said shaft.
 10. The moulding unit according to claim 9, wherein said ends of the two half-moulds (17 a, 17 b) have corresponding fingers embracing the shaft and aligned in an alternating fashion along the vertical length of the shaft. 